ADDITIVE EFFECT OF TREADMILL EXERCISE AND 17-BETA-ESTRADIOL REPLACEMENT ON PREVENTION OF TIBIAL BONE LOSS IN ADULT OVARIECTOMIZED RAT

The effects of 17beta-estradiol (E2) and treadmill exercise on tibial bone mass, tibial uptake of Ca-45, and proximal tibia osteoblast and o steoclast cell number were determined in adult ovariectomized rats. Fe male rats aged 10 months were ovariectomized and divided into five gro ups: (1) sham-operated; (2) ovariectomized; (3) ovariectomized, given 10 mug E2 biweekly; (4) ovariectomized, trained to exercise on a tread mill daily; and (5) ovariectomized, given E2 and exercised. E2 and/or exercise interventions were started 2 months following surgery and con tinued for 4 months. The calcium content of the tibial metaphysis and diaphysis and the proximal cancellous bone (BV/TV) were lower in the o variectomized than in the sham-operated controls 6 months after ovarie ctomy. This lower bone content was associated with a greater bone upta ke of Ca-45 and a greater number of osteoblasts and osteoclasts in the proximal tibia compared to the control rats. The metaphyseal calcium content was higher and the Ca-45 uptake and osteoblast and osteoclast number were lower in the E2-treated rats than in the nontreated rats. In the exercised group, higher diaphyseal calcium content and proximal cancellous bone were associated with lower bone resorption parameters without a significant effect on bone formation. This study demonstrat es that E2 primarily influences tibial cancellous bone of the ovariect omized rat and a positive adaptation to exercise occurs in both cancel lous and cortical bone. Under estrogen deficiency, E2 replacement supp resses increased bone formation and resorption; exercise suppresses ma inly bone resorption. The effects of E2 replacement and exercise train ing are independent and additive.